LONDON (Thomson Reuters Foundation) - Dozens of migrants are at risk of freezing to death in Europe after heavy snowfall and bitterly cold temperatures hit Greece and the Balkans, aid agencies said on Wednesday.

Central and southeastern Europe have been gripped by freezing weather and snowstorms for days, with night-time temperatures dropping below minus 20 degrees Celsius (minus 4 Fahrenheit) in some areas.

The Missing Migrants Project, set up by the U.N. migration agency (IOM), has recorded 11 migrants deaths in the Mediterranean since the start of the year, including four who died of hypothermia at Europe's land borders with Turkey.

In the past 10 days a Somali woman was found dead from suspected hypothermia in Bulgaria and an Afghan man died in northern Greece, while last Friday two Iraqi men were found frozen to death near the Turkish-Bulgarian border, according to local media reports compiled by the project.

As refugees and asylum seekers continue to pour into Europe, poor conditions in crowded migrant camps and informal settlements are putting their health at risk, Help Refugees said.

"The snowstorms are still going on and the situation is getting increasingly desperate as thousands are exposed to freezing conditions," Help Refugees co-founder Josie Naughton said by phone from Thessaloniki in northern Greece.

The IOM said it was particularly concerned about more than 15,500 migrants and asylum seekers housed in camps without adequate facilities on the Greek islands, including many in places that have experienced heavy snowfall this week.

"DEADLY CONSEQUENCES"

The IOM's comments echo those of the U.N. refugee agency (UNHCR) which said last week migrants stranded on the islands should be quickly transferred to the mainland or other European countries.

More than 7,500 people are also stranded in freezing conditions in Serbia, including dozens trapped near the Hungarian border..

Medical charity Medecins Sans Frontieres (MSF) told British broadcaster ITV on Tuesday it had treated the first cases of frostbite in Serbia and warned of "deadly consequences" due to hypothermia if people are not given adequate winter shelter.

In the capital Belgrade about 1,500 migrants, including hundreds of unaccompanied children as young as 10, are sleeping rough in abandoned buildings, Save the Children said.

The agencies urged European governments to act rapidly to avoid further loss of life.

"We call on governments, large organizations and international agencies to reassess their bureaucratic procedures and spend money where it's needed to prevent further loss of life," said Help Refugee's Naughton.

IOM Director General William Lacy Swing said in a statement governments needed to respond to the dangers exposed by extreme weather conditions with food aid, shelter and other resources.

Andreas Ring, Save the Children's Balkans representative, said European Union (EU) member countries had a duty to ensure safe migration routes for migrants.

"The EU's failure to respond is leaving thousands of refugees and migrants, including unaccompanied children, literally out in the cold," Ring said in a statement.